Kinetics of urea synthesis and alanine uptake by perfused rat livers

Eight livers of 200 g rats were isolated and perfused in a single pass system with a semi-synthetic medium to which alanine was added to concentrations from 0.5 to 15 mmol/l. In each liver, 4-5 sets of urea synthesis rate and alanine uptake rate at different alanine concentrations were measured. The urea synthesis rate in relation to the alanine concentration was compatible with substrate inhibition kinetics. The kinetic constants were (mean +/- SD): Vmax = 10.34 +/- 3.41 mumol urea-N/(min X 100 g b.w.), Km = 1.56 +/- 0.67 mmol/l, and Ki = 5.35 +/- 2.44 mmol/l. The alanine uptake rate followed Michaelis-Menten kinetics with the constants (mean +/- SD) Vmax = 7.51 +/- 1.68 mumol/(min X 100 g b.w.) and Km = 2.14 +/- 1.04 mmol/l. The constants were assessed by non-linear iterative regression analysis. Urea synthesis exceeded alanine uptake at alanine concentrations below 2 mmol/l, and was smaller at higher concentrations. In two experiments, alanine metabolites were measured. The glucose production rate in relation to the alanine concentration suggested substrate inhibition. At high alanine concentrations, ammonia, lactate and pyruvate were released by the livers. The results indicate that whole liver urea synthesis and gluconeogenesis is inhibited by high blood alanine concentrations, in contrast to alanine uptake.